Development Of Third Harmonic Generation As A Short Pulse Probe Of Shock Heated Material

We are studying high-pressure laser produced shock waves in silicon (100). To examine the material dynamics, we are performing pump-probe style experiments utilizing 600 ps and 40 fs laser pulses from a Ti:sapphire laser. Two-dimensional interferometry reveals information about the shock breakout, while third harmonic light generated at the rear surface is used to infer the crystalline state of the material as a function of time. Sustained third harmonic generation (THG) during a similar to 100 kbar shock breakout indicate that the rear surface remains crystalline for at least 3 ns. However, a decrease in THG during a similar to 300 kbar shock breakout suggests a different behavior, which could include a change in crystalline structure.Mechanical Engineerin

Femtosecond Pump-Probe Diagnostics Of Preformed Plasma Channels

We report on recent ultrafast pump-probe experiments 28 in He plasma waveguides using 800 nm, 80 fs pump pulses of 0.2 x 1018 W/cm2 peak guided intensity, and single orthogonally-polarized 800 nm probe pulses with similar to0.1% of pump intensity. The main results are: (1) We observe frequency-domain interference between the probe and a weak, depolarized component of the pump that differs substantially in mode shape from the injected pump pulse; (2) we observe spectral blue-shifts in the transmitted probe that are not evident in the transmitted pump. The evidence indicates that pump depolarization and probe blue-shifts both originate near the channel entrance.Physic

Design of Pneumatic Diffuser System

During non-generation periods, leakage through the wicket gates of a hydroturbine often results in very poor quality water (low or zero dissolved oxygen) in the tailrace of the hydropower facility. Generally, the leakage rate is relatively small, usually about 5-10 cfs per turbine. A bottom-mounted diffuser system was designed based on laboratory-measured and manufacturer-supplied specifications about the gas transfer characteristics of the bubble plume generated by an 11- inch flexible head diffuser. The design criteria and the overall effectiveness of the system were evaluated in field tests at Lake Eufaula, Oklahoma. The analysis of field data is reported herein

Ultrafast terawatt laser sources for high-field particle acceleration and short wavelength generation

The Laser Sources working group concerned itself with recent advances in and future requirements for the development of laser sources relevant to high-energy physics (HEP) colliders, small scale accelerators, and the generation of short wave-length radiation. We heavily emphasized pulsed terawatt peak power laser sources for several reasons. First, their development over the past five years has been rapid and multi-faceted, and has made relativistic light intensity available to the advanced accelerator community, as well as the wider physics community, for the first time. Secondly, they have strongly impacted plasma-based accelerator research over the past two years, producing the first experimental demonstrations of the laser wakefield accelerator (LWFA) in both its resonantly-driven and self-modulated forms. Thirdly, their average power and wall-plug efficiency currently fall well short of projected requirements for future accelerators and other high average power applications, but show considerable promise for improving substantially over the next few years. A review of this rapidly emerging laser technology in the context of advanced accelerator research is therefore timely

Time-Resolved Electron Temperature Measurement in a Highly-Excited Gold Target UsingFemtosecond Thermionic Emission

We report direct measurement of hot-electron temperatures and relaxation dynamics for peak electron temperatures between 3400 and 11 000 K utilizing two-pulse-correlation femtosecond (fs) thermionic emission. The fast relaxation times (\u3c1.5 ps) are described by extending RT characterizations of the thermal conductivity, electron-phonon coupling, and electronic specific heat to these high electron temperatures

Using arts‐based methods to develop service user led learning materials for social work education

Theories transmitted to students about professional practice in health and social care come traditionally from the perspective of practitioners, often as a heroic narrative, inevitably casting the service‐user in a passive role (Wiltshire, 2006). Post‐modern theorists like Foucault (1973) call this the authoritative 'gaze' that objectifies the user of public services, generating a power relationship between the professional and the service user. New perspectives are needed to redress this imbalance, drawing on the narratives of the service‐users themselves (Greenhalgh, 2006). Since 2007, a project led by service‐users at London South Bank University has been experimenting with new approaches to developing learning and teaching materials for social work education using opportunities afforded by the recent accreditation of awards within the General Social Care Council’s new Post Qualifying Framework. This paper outlines some of our experiences to date of experimenting with creative arts‐based approaches to facilitate the development of learning materials as ‘inherently creative acts’ (Sagan, 2007). This paper documents the experiences of a small group of service users and academics as we have all worked in different ways to co‐produce it using different mediums and media in order to contribute our views

Computational methods and software systems for dynamics and control of large space structures

The deployment, assembly and mission-oriented maneuvering of space structures in orbit will trigger large motions of flexible, truss-type structures. In addition, the presence of on-board controls both for attitude stabilization and specified vibration tolerance requirements may further complicate the dynamic behavior of the orbiting structures. Because of safety and cost considerations, the dynamic response of the combined structural and control systems must be predicted reliably. This need can only be met through the development of reliable and efficient simulation capabilities, since there is general agreement that on-orbit experiments should be limited because of cost, time and facility constraints. The long-term objective of this research effort is to develop a next-generation computer simulator for the dynamics and control of large space structures. The simulator will be based on integrating four research thrusts: a new multibody dynamics formulation methodology, modeling capabilities in long/slender truss-beam components with realistic joints, efficient computational procedures that can be implemented either in sequential or concurrent computers, and prototype simulation modules that can be easily processed into a modern large-scale engineering software system such as the NASA/Computational Structural Mechanics (CSM) testbed

Plasma Channels And Laser Pulse Tailoring For Gev Laser-Plasma Accelerators

We have demonstrated distortion-free guiding of I TW pulses at near relativistic intensity (0.2 x 10(18) W/cm(2)) over 60 Rayleigh lengths at 20 Hz repetition rate in a preformed helium plasma channel. As steps toward efficient channeled Laser Wakefield Acceleration up to the dephasing limit, we have upgraded our laser system from I to 4 TW, adapted femtosecond interferometric diagnostics to probe plasma density fluctuations inside the channel, and developed detailed strategies for managing ionization distortions at the channel entrance and exit at the upgraded intensity. We also report simulations, and preliminary experiments, that explore a strategy for Raman-seeding laser pulses to coherently control both unchanneled and channeled LWFA in order to lower the laser energy threshold and increase the repetition rate of election pickup and acceleration.Physic

Laser-plasma harmonics with high-contrast pulses and designed prepulses

One aspect of the complexity of mid- and high-harmonic generation in high-intensity laser-plasma interactions is that nonlinear hydrodynamics is virtually always folded together with the nonlinear optical conversion process. We have partly dissected this issue in picosecond and subpicosecond interactions with preformed plasma gradients, imaging and spectrally resolving low- and mid-order harmonics. We describe spatial breakup of the picosecond beam in preformed plasmas, concomitant broadening and breakup of the harmonic spectrum, presumably through self-phase modulation, together with data on the sensitivity of harmonics production efficiency to the gradient or extent of preformed plasma. Lastly, we show preliminary data of regular Stokes-like and anti-Stokes-like satellites to the harmonics, accompanied by modification of the forward-scattered beam. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87448/2/342_1.pd